Bronchiolitis is the #1 cause of hospitalization in US infants, with ~130,000 hospitalizations annually. Small cohort studies (n<210) suggest that 40-50% of infants hospitalized with bronchiolitis will later develop asthma. The greatest challenges for developing primary prevention strategies for this large group of children (indeed, all children) are the very early identification of modifiable risk factors and the heterogeneity of asthma. The 35th Multicenter Airway Research Collaboration (MARC-35) study (U01AI-087881; Camargo, PI) is a 17-center prospective cohort study that completed enrollment of 921 hospitalized infants with bronchiolitis in 2014. In this diverse cohort (53% African-American or Hispanic), investigators have collected biospecimens, including nasal swabs at the index hospitalization (median age 3 months) and at an age 3y exam (R01AI-114552; Camargo, PI). Follow-up data include biannual parent interviews and medical records to age 5 years, with ~90% follow-up to date. This UG3/UH3 application would: 1) extend the largest severe bronchiolitis cohort in the world (e.g., through an age 6y in-person exam to diagnose and phenotype asthma), and 2) build on our local MARC-43 cohort of 120 healthy infants by adding 600 healthy infants from four diverse sites (total n=720). Both cohorts undergo similar procedures (e.g., serial nasal swabs during early childhood). Our overarching hypothesis is that airway Moraxella abundance is associated with increased risk of childhood asthma, and our pilot data are supportive. In Aim 1, we will investigate the relation of the airway microbiome in early infancy to risk of age 6y asthma among infants with severe bronchiolitis (MARC-35). In Aim 2, we will do the same but among healthy infants (MARC-43). In Aim 3, we will investigate the relation of longitudinal patterns of the airway microbiome (e.g., infancy, age 3y, age 6y) to risk of childhood asthma in the two cohorts combined. In a subset of 200 children (100 from each cohort), we will use whole genome shotgun (WGS) sequencing to examine the relations of bacterial species and metabolic potential in early infancy to risk of asthma. For all 3 Aims, we will examine if associations differ by asthma phenotype (e.g., allergic asthma). The investigators are NIH-funded researchers working in an outstanding research environment. The proposed project is innovative and, by providing a strong evidence base for the future development of targeted microbiome interventions, advances research on the primary prevention of asthma. Moreover, the two racially/ethnically-diverse cohorts will provide the ECHO Consortium with comprehensive data on demographics, development, environmental exposures, genetics, and outcomes from two already-harmonized, multicenter U.S. studies.